Method and apparatus for deploying a liquid metal thermal interface for chip cooling

ABSTRACT

In one embodiment, the present invention is a method and apparatus for chip cooling. One embodiment of an inventive method for bonding a liquid metal to an interface surface (e.g., a surface of an integrated circuit chip or an opposing surface of a heat sink) includes applying an adhesive to the interface surface. A metal film is then bonded to the adhesive, thereby easily adapting the interface surface for bonding to the liquid metal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/220,878, filed Sep. 6, 2005, which in turn claims the benefit of U.S.Provisional Patent Application Ser. No. 60/637,100, filed Dec. 17, 2004,and U.S. Provisional Patent Application Ser. No. 60/637,117, also filedDec. 17, 2004, all of which are herein incorporated by reference intheir entireties.

REFERENCE TO GOVERNMENT FUNDING

The present invention was made with Government support under ContractNo. H98230-04-C-0920, awarded by the Maryland Procurement Office. TheGovernment has certain rights in this invention.

BACKGROUND

The invention relates generally to semiconductor devices, e.g,integrated circuits, and relates more particularly to the cooling ofintegrated circuit chips. Specifically, the present invention relates toa thermal interface for chip cooling.

Efficient cooling of integrated circuit (IC) devices is essential toprevent failure due to excessive heating. Efficient cooling of the ICchips depends in large part on good contact between the chips and theheat sinks or thermal spreaders, because a major part of the heatresistance budget is expended between the chip and the heat sink.

Conventionally, heat transfer between a chip and a heat sink isfacilitated by providing a thin layer of thermally conductive paste orgrease disposed between opposing surfaces of the chip and the heat sinkunit. Typically, the layer of paste is approximately 100 microns thickand is mechanically compliant to conform to the sometimes irregularsurfaces of the chip and heat sink.

Such conductive pastes have generally proven to be reliable infacilitating heat transfer; however, the thermal conductivity ofconventional pastes is typically limited (e.g., typical pastes have athermal resistance of approximately 10 to 100 mm²-° C./W). Thus, thesepastes are only practical for use with relatively low-power IC chips.Moreover, heavy thermal cycling may cause non-uniform behavior inconventional pastes, or may cause conventional pastes to fail tothermally bond the chip to the heat sink, resulting in thermal run-awayand also limiting chip cooling.

Thus, there is a need for a thermal interface that is capable ofestablishing reliable thermal contact, and of providing sufficientthermal conductivity and mechanical compliance between a semiconductordevice and a heat sink.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is a method and apparatus forchip cooling. One embodiment of an inventive method for coupling orbonding a liquid metal material to an interface surface (e.g., a surfaceof an integrated circuit chip or an opposing surface of a heat sink)includes applying an adhesive to the interface surface. A metal film isthen bonded to the adhesive, thereby easily adapting the interfacesurface for bonding to the liquid metal material. The metal film andadhesive are adapted to facilitate adhesion of the liquid metal materialto the integrated circuit chip and/or to the heat sink, as well as toprovide good wetting with respect to the liquid metal material and toprovide a barrier that isolates the liquid metal material from theintegrated circuit chip and/or heat sink materials.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe obtained by reference to the embodiments thereof which areillustrated in the appended drawings. It is to be noted, however, thatthe appended drawings illustrate only typical embodiments of thisinvention and are therefore not to be considered limiting of its scope,for the invention may admit to other equally effective embodiments.

FIG. 1 is a schematic diagram illustrating one embodiment of a system inwhich a thermal interface according to the present invention isdeployed;

FIG. 2 is a cross-sectional view of the thermal interface illustrated inFIG. 1; and

FIG. 3 is a flow diagram illustrating one embodiment of a method forapplying at least part of a liquid metal (or solder) thermal interface,such as the thermal interface illustrated in FIGS. 1 and 2, to a surfaceof an IC chip or heat sink.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

In one embodiment, the present invention is a liquid metal thermalinterface that facilitates improved thermal contact between asemiconductor device (e.g., an IC chip) and a heat sink overconventional conductive paste interfaces. The thermal interface isassembled in a manner that substantially prevents reactions betweenmaterials in the liquid metal and materials in the IC chip and/or heatsink (e.g., such as corrosion of heat sink materials), as well asadheres the liquid metal to the surfaces of the IC chip and/or heat sinkand achieves sufficient wetting of the IC chip and heat sink surfaces(wetting of the IC chip more significant than wetting of the heat sinkin many embodiments). Thus, the thermal interface of the presentinvention makes deployment of the liquid metal a practical interfacesolution.

FIG. 1 is a schematic diagram illustrating one embodiment of a system100 in which a thermal interface 102 according to the present inventionis deployed. As illustrated, the thermal interface 102 is deployedbetween opposing surfaces of an IC chip 104 and a heat sink 106 in orderto provide good thermal contact between the IC chip 104 and the heatsink 106. The heat sink 106 may be any kind of thermal heat sink orspreader, including an air-cooled fin assembly, a liquid-cooledassembly, a heat pipe assembly, a vapor chamber and the like. In oneembodiment, the thermal interface 102 includes a thermally conductiveliquid metal material, as described in further detail below. Inalternative embodiments, the thermal interface 102 may be a solderthermal interface.

FIG. 2 is a cross-sectional view of the thermal interface 102illustrated in FIG. 1. In one embodiment, the thermal interface 102includes a thermally conductive liquid metal layer 200 disposed betweena first barrier layer 202 a in contact with a first surface of theliquid metal layer 200 and a second barrier layer 202 b in contact withan opposite second surface of the liquid metal layer 200 (hereinaftercollectively referred to as “barrier layers 202”).

The liquid metal layer 200 is adapted to facilitate good thermal contactbetween the opposing surfaces of the IC chip 104 and the heat sink 106.The liquid metal layer 200 comprises a metal (or metal alloy) that is aliquid at least in the range of typical IC chip operation temperatures(e.g., approximately 20° C. to 100° C.), and in one embodiment comprisesa gallium-indium-tin alloy that has an even higher thermal conductivity(approximately 30 W/m-K) than conventional conductive pastes. In furtherembodiments materials may be substituted or added to the liquid metalalloy in order to adjust one or more properties of the liquid metalalloy, such as its melting point, corrosion resistance, conductivity orwetting. For example, materials including, but not limited to, zinc,bismuth, platinum, palladium, manganese, magnesium, copper, silver orgold may be added or may substitute for different materials.

In further embodiments still, the liquid metal layer 200 comprises agallium-indium-tin alloy that is mixed with inert particles (e.g.,organic particles such as tungsten, carbon, diamond, silicon dioxide,silicon carbide, chromium, titanium, molybdenum, gallium oxide, tinoxide, indium oxide, plastic tantalum and the like) to enhance theviscosity of the liquid metal layer 200. For example, ten weight percentof approximately two micron size tungsten particles may be combined withthe gallium-indium-tin alloy to produce a more viscous, slurry-likeliquid metal layer 200 that is easier to apply and more easilycontained. In other embodiments, the type, size and weight percent ofinert particles added to the gallium-indium-tin alloy is dependent uponthe goals of the specific application (e.g., desired bond line, etc.).Additionally, anti-corrosive materials such as zinc, palladium,platinum, gold, manganese and magnesium may be added to thegallium-indium-tin alloy in small amounts in order to adjust corrosionproperties of the gallium-indium-tin alloy.

In one embodiment, the liquid metal layer 200 has a thickness ofapproximately ten to one hundred microns. In other embodiments, thethickness of the liquid metal layer will depend upon the desired thermalresistance and/or on the mechanical tolerances of the components.Typically, the thickness of the liquid metal layer 200 is directlyproportional to the thermal resistance. For example, in one embodiment,the liquid metal layer 200 is approximately thirty microns thick andcorresponds to a thermal resistance of approximately two mm²-° C./W.

The barrier layers 202 are adapted to bond the liquid metal layer 200 tothe opposing surfaces of the IC chip 104 and the heat sink 106 whilesubstantially preventing reactions between components of the liquidmetal layer 200 and components of the IC chip 104 and the heat sink 106(e.g., such as corrosion of heat sink 106). In one embodiment, thebarrier layers 202 are applied directly to the opposing surfaces of theIC chip 104 and the heat sink 106 (e.g., by evaporation, sputtering,plating, bonding and the like). The barrier layers 202 are at leastpartially formed of a metal-containing material that has a lowsolubility in the constituents of the liquid metal layer 200 (e.g.,gallium, indium and tin) and is chemically unreactive. For the purposesof the present invention, a material that has a “low solubility” in theconstituents of the liquid metal layer 200 is a material that has a lowenough solubility in the liquid metal layer 200 that the material canreasonably be expected to maintain its integrity over time within atemperature range of interest.

In one embodiment, the materials that form the barrier layers 202 alsoadhere well to copper, aluminum, silicon, silicon nitride and silicondioxide (e.g., common heat sink and/or IC chip materials). In oneembodiment, the barrier layers 202 are at least partially formed of amaterial containing at least one of: chromium, tantalum, titanium,tungsten, molybdenum and nickel. In further embodiments, the barrierlayers 202 are at least partially formed of a material containing anoxide or nitride such as: silicon oxide, silicon nitride, siliconcarbide, titanium nitride, and tantalum nitride. In such cases, it maybe desirable to include an additional adhesion layer (e.g., chromium)deposited on the interface surfaces between the nitride or oxide barrierlayer 202 and the interface surfaces (e.g., deposited on the interfacesurfaces prior to applying the barrier layer 202). In one embodiment,the barrier layers 202 each have a thickness of approximately 2000 to5000 Angstroms.

In one embodiment, the thermal interface 102 further includes at leastone wetting layer 204 a or 204 b (hereinafter collectively referred toas “wetting layers 204”) disposed between the liquid metal layer 200 andone or both of the barrier layers 202. The wetting layers 204 areadapted to facilitate a metal-to-metal bond between the liquid metallayer 200 and the barrier layers 202. The wetting layers 204 are atleast partially formed of a small quantity of material that optionallydissolves partially or completely in the constituents of the liquidmetal layer 200 (e.g., gallium, indium and tin) to permit a direct,robust metal bond between the liquid metal layer 200 and the barrierlayers 202.

In an alternative embodiment, the wetting layers 204 may be at leastpartially formed of a material that wets directly (such as siliconcarbide, silicon nitride or silicon oxide), instead of a material thatfacilitates a metal-to-metal bond between the liquid metal layer 200 andthe barrier layers 202, as described above. In the context of thepresent invention, a material that “wets directly” is a material thathas good surface energy with respect to the liquid metal material suchthat a wetting layer that facilitates a metal-to-metal bond is notnecessary

Moreover, contents of the wetting layers 204 should be selected from agroup of materials that do not adversely affect the function of theliquid metal layer 200, e.g., by reacting chemically with or alteringthe alloy properties of the liquid metal material. In one embodiment,the wetting layers 204 are directly applied to the barrier layers 202 ina manner that substantially prevents oxides from forming on the barrierlayers 202 (e.g., by sputtering). In one embodiment, the wetting layers204 are at least partially formed of at least one noble metal (e.g.,platinum or gold) that dissolves into the liquid metal layer 200 oncontact, permitting a direct metallic bond between the liquid metallayer 200 and the barrier layers 202. In one embodiment, each of thewetting layers 204 has a thickness that is small enough to avoidadversely affecting the properties of the liquid metal material. In oneembodiment, the thickness of the wetting layers 204 is approximately 300hundred Angstroms, but could potentially be thinner or thicker asnecessary. For example, the surface roughness of the IC chip or heatsink surface may dictate the thickness of the wetting layer 204 (e.g., arougher surface might require a thicker wetting layer 204 that a smoothsurface would, in order to achieve an oxygen impervious coating).

In one embodiment, at least one of the barrier layers 202 and thecorresponding wetting layer 204 could be formed as a single, combinationbarrier and wetting layer, for example formed at least partially ofchromium or nickel.

Those skilled in the art will appreciate that in some embodiments, thefirst barrier layer 202 a and the second barrier layer 202 b, as well asthe individual wetting layers 204, may comprise different materials andthicknesses depending upon whether the layer(s) in question are adaptedto interface with the IC chip 104 or with the heat sink 106.

Those skilled in the art will recognize that a variety of methods may beimplemented for laterally confining the liquid metal layer 200 (e.g., toprevent the liquid metal material from “leaking”). In one embodiment,lateral confinement of the liquid metal layer 200 is accomplished bysmall gap capillary action. In another embodiment, the liquid metallayer 200 is laterally confined by a gasket interposed between oragainst the IC chip 104 and the heat sink 106. In yet anotherembodiment, the first and second barrier layers 202 are joined at theedges to form an enclosure to contain the liquid metal layer 200.

The present invention thereby provides a practical liquid metal thermalinterface 102 that is easily deployed between an IC chip 104 and a heatsink 106. The barrier layers 202 prevent the material in the liquidmetal layer 200 from reacting with the materials that make up the heatsink 106 and the IC chip 104. Moreover, the wetting layers 204 easilydissolve into the liquid metal layer 200, thereby facilitating directmetal-to-metal contact between the liquid metal layer 200 and thebarrier layers 202. Thus, the liquid metal thermal interface 102 can bedeployed in a manner that avoids adverse chemical reactions with andachieves sufficient wetting of both the IC chip 104 and the heat sink106, while providing improved thermal contact over conventionalconductive pastes.

For example, in one embodiment, an approximately 2000 Angstrom barrierlayer 202 formed of chromium is sputtered onto the IC chip 104 and/orheat sink 106 and subsequently sputter coated with a wetting layer 204comprising approximately 300 Angstroms of gold or platinum to preventsurface oxidation. The liquid metal layer 200 is then deployed betweenthe coated IC chip 104 and heat sink 106. In one embodiment, thisconfiguration achieves robust performance over temperature ranges up toapproximately 150° C.

In another embodiment, approximately 300 Angstroms of chromium, 2500Angstroms of titanium, tungsten, or tantalum nitride-coated tantalum,and 300 Angstroms of platinum or gold are successively sputtered ontothe surface of the IC chip 104 and/or heat sink 106. In one embodiment,this configuration provides enhanced resistance to gallium, indium andtin diffusion at high temperatures (e.g., temperatures in excess ofapproximately 150° C.

However, those skilled in the art will appreciate that selected methodsfor coating the surfaces of the IC chip 104 and heat sink 106 with thebarrier and/or wetting layers 202 and 204, as well as the selectedthicknesses of the barrier and wetting layers 202 and 204, are typicallyapplication-dependent. That is, coating methods and thicknessestypically will depend on deposition stresses in the barrier and wettinglayers 202 and 204 and on desired coating uniformity. For example, ifsurface of the IC chip 104 or heat sink 106 is rough, the barrier and/orwetting layers 202 and 204 may be applied by sputtering to achieve asubstantially uniform coating. Application by sputtering also tends tocause less stress in the film than other application methods. In someembodiments, a thin seed layer for a barrier and/or wetting layer 202and 204 may be applied in accordance with an appropriate coating method,followed by a thicker layer plated thereon.

FIG. 3 is a flow diagram illustrating one embodiment of a method 300 forapplying at least part of a liquid metal (or solder) thermal interface,such as the thermal interface 102 illustrated in FIGS. 1 and 2, to asurface of an IC chip or heat sink. Specifically, FIG. 3 illustrates amethod 300 for applying a barrier and/or wetting layer to an interfacesurface (e.g., the surface of the IC chip 104 or the heat sink 106 thatis to be interfaced via the thermal interface 102). The method 300 isinitialized at step 302 and proceeds to step 304, where a bonding stripcomprising a wetting layer 206 (e.g., comprising a wetting material suchas platinum or gold) and/or a barrier layer 204 (e.g., comprising abarrier material such as titanium, tantalum, tungsten, chromium ornickel) is prepared. In one embodiment, a bonding strip is formed bysuccessively evaporating a wetting layer 206 and a barrier layer 204onto a polymer backing film (e.g., a polyethylene terephthalate filmsuch as Mylar® of El Dupont de Nemours & Company or apolytetrafluoroethylene film such as Teflon®, also of El Dupont deNemours & Company) to form a two-layer metal film that coats at least aportion of the polymer backing film. In an alternative embodiment, acombination barrier/wetting layer or metal film (e.g. comprisingchromium) is evaporated onto the polymer backing film to form asingle-layer metal film. In one embodiment, the thickness of the metalfilm is on the order of at least approximately 1000 Angstroms.

In yet another alternative embodiment, a bonding strip is formed bystripping a barrier foil (e.g., a metal foil formed of a barriermaterial) of oxides and then coating a surface of the stripped foil witha wetting material to form a two-layer metal film. In one embodiment,the metal film has a thickness of approximately one to twenty microns.

In one embodiment, the barrier layer of the bonding strip is furthercoated with an adhesion promoter (e.g., an organic adhesion promotersuch as a commercially available silated or non-silated promoter).

In step 306, the interface surface of the IC chip 104 or the heat sink106 is prepared for bonding. In one embodiment, preparation of theinterface surface involves applying an adhesive to the interfacesurface. In one embodiment, the applied adhesive is a material thatfacilitates good adhesion of the interface surface to the bonding strip,that is stable, that is capable of forming a thin (e.g., as thin assub-micron) bond line, and that can tolerate elevated temperatures(e.g., up to approximately 250° C.) to allow solder reflow operations.In one embodiment, the adhesive comprises a commercially available lowviscosity acrylate or epoxy or a low-melt solder (e.g., an indium-basedsolder). In further embodiments, an adhesion promoter having one or moreof the same requisite properties as the adhesive itself may be appliedto the interface surface prior to applying the adhesive.

In step 308, the bonding strip (e.g., the coated polymer backing film orthe coated barrier foil) is pressed to the interface surface so that thebarrier layer 204 of the metal film on the bonding strip contacts theadhesive on the interface surface. In one embodiment, the adhesive ispressed to a bond line of approximately 1000 Angstroms (e.g., by rollingor other suitable means). A small bond line allows the interface toretain high thermal conductivity.

In step 310, the adhesive is cured (e.g., by heat, light or a thermalreflow cycle), bonding the barrier layer (and corresponding wettinglayer) directly to the interface surface. In one embodiment, theadhesive is cured at a temperature of at least approximately 250° C. Inone embodiment (e.g., where the bonding strip comprises a coated polymerbacking film) the method 300 proceeds to step 312 and removes thepolymer backing film, leaving behind the metal film (e.g., the barrierand wetting layers) bonded to the interface surface. Steps 304-312 maybe repeated any number of times to bond any number of metal films to theinterface surface. The method 300 terminates at step 314.

Thus, a surface of an IC chip or a heat sink (or other surface requiringthermal interface to another object) may be prepared for directapplication of a liquid metal or solder thermal interface material (suchas a gallium-indium-tin alloy or other thermally conductive metal-basedinterface material). The method 300 enables deployment of a liquid metal(or solder) thermal interface in a manner that is, in some embodiments,easier and more cost effective than sputtering, plating or evaporatingbarrier and/or wetting layers directly onto an interface surface.

Thus, the present invention represents a significant advancement in thefield of IC chip cooling. A liquid metal thermal interface is providedthat facilitates improved thermal contact between an IC chip and a heatsink over conventional conductive paste interfaces. The thermalinterface is assembled in a manner that substantially prevents reactionsbetween materials in the liquid metal and materials in the IC chipand/or heat sink, as well as achieves sufficient wetting of the IC chipand heat sink surfaces. Thus, the thermal interface of the presentinvention makes deployment of the liquid metal a practical interfacesolution. Moreover, the present invention provides a simple,cost-effective method of deploying the liquid metal thermal interfacebetween an IC chip and a heat sink (or between any two surfacesrequiring good thermal contact).

While foregoing is directed to the preferred embodiment of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A metal film for bonding a liquid metal to an interface surface, theinterface surface comprising a surface of an integrated circuit chip ora surface of a heat sink, said metal film comprising: a layer of barriermaterial adapted to bond to said interface surface to said liquid metal,said barrier material having a low solubility in said liquid metal; anda layer of wetting material coating at least a portion of said barriermaterial and adapted to bond said layer of barrier material to saidliquid metal, said wetting material being at least partially dissolvablein constituents of said liquid metal.
 2. The metal film of claim 1,wherein said layer of wetting material is disposed on a polymer backingfilm that is removable from said metal film.
 3. The metal film of claim1, wherein said layer of barrier material and said layer of wettingmaterial comprise a single layer of material having properties of bothof said barrier material and said wetting material.
 4. The metal film ofclaim 1, wherein said barrier layer is formed of at least one of:titanium, tantalum, tungsten, chromium, molybdenum, silicon, siliconcarbide, silicon nitride, silicon dioxide, titanium nitride, tantalumnitride or nickel.
 5. The metal film of claim 1, wherein said wettinglayer is formed of at least one noble metal.
 6. The metal film of claim1, wherein said wetting layer is formed of at least one of: platinum orgold.
 7. The metal film of claim 1, wherein said metal film has athickness of at least approximately 1000 Angstroms.
 8. The metal film ofclaim 1, wherein at least one of the barrier layer and the wetting layeris evaporated onto a polymer backing film.
 9. The metal film of claim 8,wherein the polymer backing film is formed of a polyester film.
 10. Themetal film of claim 8, wherein the polymer backing film is formed of aat least one of: polyethylene terephthalate or polytetrafluoroethylene.